1. Field
This invention relates to devices for sealing of rotating drum kilns against entry of atmospheric air, and more specifically to rotating seals for that purpose. Still more specifically the invention relates to such seals for kilns utilized to regenerate spent activated carbon.
2. State of the Art
"Activated" carbon is used in many chemical processes for absorption of materials, such as are often contained in liquid solutions. With repeated use, the carbon becomes saturated with the contaminant and loses its ability to absorb it. In this event, the carbon is no longer activated and must either be discarded or reactivated to remove the unwanted contaminants from the surface of the carbon and its pore structure. To reactivate such carbon, it is heated to high temperatures, ranging from 550.degree. C. to 800.degree. C., in air or in a steam rich atmosphere. Efficient treatment of the spent carbon requires it to be turned and tumbled, to provide access of the heat and steam to all of the carbon and all of its pores. This is typically done in a kiln having a rotating drum into which the carbon to be treated is fed. In some installations, steam is injected, along with fuel and air for internal heat producing combustion. In others, a portion of the exterior of the drum is heated in an oven, and steam may be provided by feeding the carbon in a damp condition and allowing the resultant steam to enrich the atmosphere the full length of the drum.
Regardless of the type of rotating kiln used, a tubular drum rotates to tumble the contents, which are generally granular in form, to provide even heating for most efficient treatment. The drum is installed at a downward slope from an inlet end to an outlet end. Baffles on the inside of the drum wall help achieve even tumbling and even heating.
The rotating drum joins with an inlet end and an outlet end stationary hood structure, the former mounting material feed apparatus. The outlet end hood directs outlet gases, primarily steam, from the drum to the atmosphere, and is connected to treated material collection containers. The junctures of rotating drum inlet and outlet ends with the associated stationary hoods requires a rotary mechanical seal device, to preclude entry of unmetered atmospheric air into the drum interior. Such incursion could result in burning of the carbon being treated. Also, maintenance of the temperature of the internal steam atmosphere required for efficient reactivation would be difficult without effective atmospheric seals.
A rotating seal component on each end of the drum must bear against a stationary seal member connected to the adjacent stationary hood. This requires mechanical seals having mating rotating and stationary annular bearing surfaces, diametrially even larger than the drum. Providing substantially constant pressure between the bearing faces is complicated by the expansion and contraction of the drum both longitudinally and diametrally with change of operating oven temperature.
In prior art seal design is, the stationary seal component is generally fixed to the respective hood, and cannot move longitudinally or diametrally to the drum. The rotating seal component, however, is joined to the outside circumference of the drum by a non-metallic membrane allowing the drum to expand and contract while the rotating seal component remains in contact with its stationary counterpart. The rotating seal member is oversized in diameter to allow for diameter increase of the drum. With this arrangement, the rotating component must itself be sealed against entry of air. Pressure between the fixed and turning seal faces is maintained by compression springs acting between a flange welded to the drum and the turning seal component. Other arrangements may employ tension springs. In either event, the spring induced normal force between the seal components must vary substantially with drum expansion and contraction, because of associated variation in spring force with length. Another shortcoming is leak producing tilting or cocking of the rotating seal, which must often float loosely upon the connecting membrane. Normal face contact pressure of the seal elements will also decrease with wear of the faces, or wear plates if employed, with prolonged use.
A critical need therefore remains for a rotary sealing device which reliably compensates for longitudinal and diametral expansion and contraction of the rotating drum, to reliably prevent the intrusion of unwanted air into the interior of the rotating drum of the kiln.